1. Field of the Invention
The present invention relates generally to processes for preparing holograms and the photosensitive hydrophilic gelatin layers which are present in holograms. More specifically, the present invention deals with methods for hardening the hologram gelatin after it has been developed and dehydrated.
2. Description of Related Art
Holograms are widely used for a variety of purposes including: helmet-mounted displays such as those disclosed in U.S. Pat. No. 3,928,108; eye protective reflectors for laser radiation; and novelty displays, such as pendant jewelry. In fabricating a hologram, a pre-holographic element comprising a hydrophilic, photosensitive layer on a substrate is processed to expose the photosensitive layer to an actinic interference pattern to record a latent image thereon. The photosensitive layer is then developed to obtain the recorded latent image followed by dehydration, drying and hardening.
The gelatins used in forming the holograms are mixtures of proteins containing both acidic and basic groups which make the gelatins hydrophilic. The gelatins can absorb up to ten times their own weight in water. The process of forming the gelatin hologram utilizes the difference in refractive index between hardened and unhardened gelatin to produce the hologram. The differential hardening of the gelatin is provided by photosensitive material such as dichromates, diazo compounds and silver halide emulsions. Dichromated gelatins are the most common types of gelatins used. Since swelling of the gelatin during processing is necessary to get the full difference in refractive index, the gelatins cannot be completely hardened prior to processing.
A common method used for sensitizing and processing dichromate gelatin holograms is described by L. H. Lin, Appl. Opt., Volume 8, No. 5, 963-966 (May 1969) and by R. G. Brandes, E. E. Francois and T. A. Shankoff, Appl. Opt., Volume 8, No. 11, 2346-2348 (November 1969). These methods typically involve preliminary hardening of the gelatin followed by sensitization of the gelatin and exposure followed by development of the hologram. The hologram is then dehydrated in increasing strengths of isopropyl alcohol followed by drying.
It is desirable to increase the resistance of the completed gelatin hologram to attack by moisture and heat. U.S. Pat. No. 4,029,507 describes a process in which the gelatin hologram is treated with formaldehyde during the dehydration step. The combination of formaldehyde with 2-propanol during the dehydration step is disclosed as providing final hardening of the fully processed dichromated gelatin hologram. In carrying out the final hardening of the gelatin hologram, it is important that the efficiency (i.e., refracted index modulation) of the hologram not be adversely affected. Accordingly, it is desirable to harden the gelatin hologram as much as possible to increase its resistance to moisture and heat while at the same time not unduly decreasing the photographic sensitivity.
There is a continuing need to provide processes for stabilizing gelatin holograms after processing which provide increased levels of moisture and heat resistance while still providing high refractive index modulation.